1. Field of the Invention
This invention relates to a novel method for preparing new homo- and co-polymers by Atom Transfer Radical Polymerization and novel compositions of homo- and copolymers thereof exhibiting narrow polydispersity index.
2. Description of the Related Art
The formation of block or graft copolymers of non-vinyl polymers with vinyl monomers by a radical mechanism, has been reported to have been achieved by two methods. One is the use of an end functional polymer which can react with an end or pendent groups of the second polymer; the second method is to use a starting step-grown polymer as a macroinitiator and grow the vinyl polymer from it, or the use of a monofunctional vinyl polymer in a step growth polymerization with AA and BB monomers.
However, both of the above methods have certain limitations. The first method requires that well defined vinyl polymers with known functionalities be made. The other method requires that functional groups must be present at the ends of the polymer (block) or dispersed along the polymer backbone (graft) which can react with those on the vinyl polymer. Also, if the vinyl polymer is not compatible with the growing polycondensation polymer the polymerization will result in incomplete formation of a block or graft copolymer and a mixture of homopolymers. In the second method, by using conventional radical polymerization, the generation of a radical at either a pendent group or at a chain end results not only in the synthesis of homopolymer, due to transfer to monomer or polymer, but also may lead to the formation of crosslinked gels.
Thus, a polymerization can be initiated by decomposition of a functional group (azo, peroxy, etc.) either in the macroinitiator's backbone or along a pendent side group, Scheme 1. Further, an irreversible activation of a functional group can take place at the polymer chain ends or attached to a pendent side group, Scheme 2. ##STR1##
The decomposition of functional groups in a macroinitiator backbone is accomplished by copolymerization of a functional monomer during the synthesis of the macroinitiator. The functional monomer contains a functional group which can decompose. These radicals can then initiate the polymerization of a vinyl monomer to form a block copolymer. If more than one functional group is present in the macroinitiator, then the chain can be broken into smaller chains which have radicals at both ends.
In the literature, there are some examples of the incorporation of azo groups in the backbone of polymer chains. Akar et al (Polym. Bull. 1986, 15, 293) and Hizal et al (Polymer, 1989, 30,722) use a difunctional cationic initiator with a central azo group. After the synthesis of a polymer by cationic polymerization, the azo group can be decomposed to form polymer chains with a radical at one end capable of initiating radical polymerization. This results in the formation of AB block copolymers.
Udea et al (Kobunshi Ronbunshu, 1990, 47,321) discusses the use of azodiols, as comonomers, in condensation polymerizations allowing for the introduction of more than one azo group per polymer chain. Decomposition of this macroinitiator in the presence of vinyl monomer results in the formation of AB block copolymers.
Azodiamines have reportedly been used (Vaslov et al, Makromol. Chemie 1982, 183,2635) as a comonomer in the ring-opening polymerization of N-carboxy anhydrides in the synthesis of polypeptides. Again, these polymers are macroinitiators which can form ABA triblocks by decomposition, followed by initiation of a radical polymerization.
ABA block copolymers have also been synthesized by macroinitiators which have azo groups at the ends of the polymer chain. These macroinitiators were synthesized by the reaction of an azo compound, which had an acid chloride functional group, with the diol end groups of polytethylene oxide) (PEO) or poly(dimethylsiloxane) (PDMS)(Harabaglu, Makromol. Chem. Rapid Commun. 1990, 11,433). Decomposition of the azo end groups resulted in either a PEO or PDMS macro radical. When this was done in the presence of a vinyl monomer, ABA polymers were synthesized. However, a radical complementary to the macroradical was also generated resulting in the formation of homopolymer.
Macroinitiators with side chain azo groups (Kerber et al., Makromol. Chem. 1979, 180,609; Nuyken et al., Polym., Bull 1989, 21,23) or peroxyester (Neckers, J. Radiat. Curing 1983, 10,19; Gupta, J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 147) groups were used in the synthesis of graft copolymers. These macroinitiators were synthesized by the use of comonomers in step-growth polymers. These systems also formed homopolymer upon decomposition of the peroxyester.
Another category of macroinitiators are those which possess a functional group that can be activated to form a radical. One such example is reported by Bamford (Bamford, New Trends in the Photochemistry of Polymers; Elsevier Applied Science Publishers, London, 1985) when trichloro polymer end groups were irradiated in the presence of manganese pentacarbonyl. In the presence of a monomer, block copolymers were formed.
Polystyrene with dimethylamino end groups, when irradiated in the presence of 9-fluorenone and a monomer, gave block copolymers (Yagci, Polymer Commun; 1990, 31,7). This was done by formation of a radical through the reaction of the dimethyl amine and the triplet state of the aromatic ketone. By analogy, graft copolymers were synthesized by using poly(styrene-co-p-N,N'-dimethylamino styrene) as the macroinitiator (Kinstle et al., J. Radiat. Curing 1975, 2,7).
Although these methods have produced block and graft copolymers, the materials that have been prepared are not well defined. In most cases, homopolymers of the vinyl monomers are formed due to transfer to monomer during the radical polymerization or because of a second radical formed during the decomposition of the azo or peroxy group, Scheme 1. In the synthesis of graft copolymers, crosslinked gels can be formed if termination of the growing vinyl polymer is by combination. The molecular weights of the grafts or blocks that are synthesized by the radical polymerizations are not well defined. Also, not all of the azo (or peroxy) groups may decompose and/or initiate polymerization during the synthesis of a block or graft copolymer. Because of incomplete initiation, the number of grafts, or length of blocks cannot be accurately predicted.
Thus, there is a need for a method to prepare block and graft copolymers that are well defined and free of homopolymer.
Further, Flory (Flory, P. J. J. Am. Chem. Soc., 1952, 74,2718) first theorized that the copolymerization of a difunctional monomer with AB.sub.2 (see definition below) monomers would lead to branched structures. In his proposal, the density of branching could be controlled by varying the relative concentration of AB.sub.2 monomer to difunctional monomer. This proposal was first put to use in the step-growth synthesis of polyphenylenes by Kim and Webster. (Webster, O. W.; Kim, Y. H. J. Am. Chem. Soc., 1990 112,4592; Webster, O. W., Kim, Y. H., Macromolecules 1992, 25,5561). Subsequently, it was extended to other step-growth polymerizations such as aromatic (Frechet, J. M. J.; Hawker, C. J.; Lee, R. J. Am. Chem. Soc. 1991, 113,4583.) and aliphatic (Hult, A.; Malmstrom, E.; Johansson, M. J. Polym. Sci. Polym. Chem. Ed. 1993, 31,619) esters, siloxanes (Mathias, L. J.; Carothers, T. W. J. Am. Chem. Soc. 1991, 113,4043) and amines (Suzuki, M.; Li, A.; Saegusa, T. Macromolecules 1992, 25,7071). Later, it was extended to cationic chain growth polymerizations by Frechet et al., (Frechet, J. M. J.; Henmi, M.; Gitsov, L.; Aoshima, S.; Leduc, M.; Grubbs, R. B. Science 1995, 269, 1080). Shortly afterwards, it was adapted to radical polymerizations by Hawker et al. (Hawker, C. J.; Frechet, J. M. J.; Grubbs, R. B.,; Dao, J., J. Am. Chem. Soc. 1995, 117, 10763) and by Gaynor et al (Gaynor, S. G.; Edelman, S. Z.; Matyjaszewski, K., ACS PMSE Preprints 1996, 74; Gaynor, S. G.; Edelman, S. Z.; Matyjaszewski, K. Macromolecules, 1996, 29,1079).
Further, polymers containing polar groups, such as polyacrylonitrile (PAN) are prepared in general by a free radical polymerization method. W. Berger et al. (Makromol. Chem., Macromol. Symp., 1986, 3, 301), describes such a free radical polymerization method for PAN. However, the free radical polymerization of acrylonitrile (AN) does not produce a polymer with well defined structure and narrow polydispersity index. Further, such free radical polymerization method is not suitable for the preparation of block copolymers.
Polyacrylonitrile has also been prepared by a polymerization method using an anionic initiator. Such a method is described by Sogah et al (Macromolecules, 1987, 20, 1473); in general, anionic polymerization provides for control of molecular weight distribution by means of the "living" nature of its propagating chain with monomers such as styrene, diene and most non-polar acrylic monomers. However, in the polymerization of monomers with polar groups, such as acrylonitrile, the carbanion initiator attacks the polar group thus losing part of the "living" nature of the polymerization method. These defects have been partly overcome by carrying out the polymerization at very low temperature; this condition, however, renders the process impractical for commercial production of polymers containing polar groups, such as PAN.
Further, Higashimura et al., (Macromolecules, 1993, 26, 744) has described "living" cationic polymerization of styrene with an initiating system based on 1-phenylethyl chloride (1-PhEtCl) and tin tetrachloride (SnCl.sub.4) in the presence of tetra-n-butyl ammonium chloride (n-Bu.sub.4 NCl) in methylene chloride as solvent. In addition, polymers with a variety of terminal functionalities can be obtained by "living" cationic polymerization and some of the end functions may be useful for initiating another polymerization to give block copolymers. Thus, well defined block copolymers by the transformation of initiating sites from "living" cationic to anionic polymerization have been described by Gadkari et al. (J. Appl. Polym. Sci., Appi. Polym. Symp., 1989, 44, 19), Liu et al. (J. Polym. Sci., A, Polym. Chem. 1993, 31, 1709); Nemes et al. (J. Macromol. Sci., 1991, A28, 311); Kennedy et al. (Macromolecules, 1991, 24, 6567); Kitayama et al. (Polym. Bull. (Berlin) 1991, 26, 513); Ruth et al. (Polym. Prepr. 1993, 34, 479); Nomura et al. (Macromolecules 1994, 27, 4853) and Nomura et al. (Macromolecules 1995, 28, 86). The disadvantage of these techniques is that they include numerous steps, and the number of monomers that can be used with any of the above-described methods is limited to those which can be polymerized by cationic or anionic methods. However, none of the prior art processes results in a polymer with as narrow polydispersity index as the present invention.
It is well known to those skilled in the art of polymers that when the polydispersity index of a polymer is wide-the polymer contains polymeric segments with substantial smaller and larger molecular weight segments than the number average molecular weight of the polymer. On the one hand, low molecular weight segments have an adverse effect on physical properties of the polymer such as tensile strength, elongation and flexural madulus; while segments of very large molecular weight result in high melt viscosity of the polymer and, thus, in inferior processability of the polymer. Thus, there is a need for a polymer with well defined and narrow polydispersity index.
Atom Transfer Radical Polymerization (ATRP) has been described by Wang et al (in J. Am. Chem. Soc., 1995, 36, 2973; and in Macromolecules, 1995, 28, 7572). However, polar monomers, such as acrylonitrile, have not been successfully polymerized by ATRP as of now.
Thus, there is a need for a method to prepare block or graft copolymers with well defined lengths and or number of blocks or grafts that can be tailor made and that a precise-number of grafts can be grown from the polymer backbone.
There is also a need for a controlled polymerization of polar monomers, such as acrylonitrile (AN) that can produce a polymer with a narrow polydispersity index and under industrially acceptable conditions.
There is also a need for polymeric materials of controlled architecture and narrow polydispersity index that may optionally contain polar groups that enhance solvent resistance properties. There is, for instance, a need for solvent resistant thermoplastic acrylate elastomers. Thermoplastic elastomers in the context of the present invention are block copolymers consisting of at least two distinct polymeric segments (blocks), which are thermodynamically incompatible and have different glass transition temperatures (Tg).